The JTM 45 was first built in 1962, handmade in an all-aluminum chassis, by Ken Bran and Dudley Craven. Because of its power, Marshall decided early on to build it as a head, with a separate 4×12″ cabinet with Celestion speakers. The amplifier itself was based on the Fender Bassman. It uses KT66 vacuum tubes or valves (though early versions had used US 5881, a version of the 6L6) in the output stage, and 12AX7 tubes (known in Britain as ECC83 valves) in the pre-amplification stage.
Significant differences between the Bassman and the JTM include the all-aluminum chassis (it is less susceptible to hum than a steel chassis), a 12AX7 valve as the first in the chain (the Bassman has a 12AY7), the Celestion speakers with a closed cabinet (compared to open-backed Jensen speakers), and a modified feedback circuit which affects the harmonics produced by the amplifier. As Ken Bran later said, “The JTM also had different harmonic content, and this was due to the large amount of feedback I had given it.” The amp was also available as a bass (which lacked a “bright” switch) and a PA version.
By the mid 1960s, the JTM 45 had become so popular that it began to supplant the ubiquitous Vox amps, even their AC50, though it was just as powerful.
In late 1965, Marshall introduced its now standard script lettering, in white, and by early 1966 it began calling the amplifiers “JTM 50”. Some 100 early models had red lettering; these are especially collectible. Other cosmetic changes included a gradual change to different knobs. The JTM 45 became the basis for many subsequent Marshalls, most notably the Bluesbreaker. It ceased being produced in 1966, but was reissued in 1989, though with a modern printed circuit board and 6L6 output valves. In 2014 Marshall reissued a “handwired” 30W amplifier based on the JTM45, the 2245THW, whose circuitry is identical to the Bluesbreaker circuit; it is a “fine high-end piece” according to Vintage Guitar, listed at $4800.